Field of the Invention
The present invention relates to an object information acquiring apparatus and a control method therefor. In particular, the present invention relates to technology for acquiring object information by transmitting acoustic waves to an object, and receiving reflected waves that are reflected within the object.
Description of the Related Art
In an ultrasonic diagnostic apparatus, which is an object information acquiring apparatus, the spatial resolution in the depth direction in the case of forming image data via the pulse-echo technique can be generally represented as (nλ)/2 when the wavelength of the ultrasonic waves is λ, and the transmission wave number is n. For instance, when two wavelengths worth of ultrasonic waves having a center frequency of 12 MHz are transmitted, the spatial resolution in the depth direction will be approximately 0.13 mm.
The pulse-echo technique is now explained. Foremost, when an ultrasonic pulse (acoustic wave pulse) is transmitted to an object, ultrasonic waves are reflected and returned according to the acoustic impedance difference within the object. Next, the reflected waves are received, and a received signal of the reflected waves is used to generate image data. Typically, an envelope of the received signal is acquired, and the acquired envelope is converted into a brightness value to generate image data. As a result of repeating the transmission and reception of ultrasonic waves to a plurality of directions or positions within the object, brightness information on a plurality of scanning lines in the direction that the ultrasonic waves were transmitted and received can be acquired. The inside of the object can be imaged by arranging the brightness information on the plurality of scanning lines.
Note that, in an ultrasonic diagnostic apparatus, a plurality of conversion elements are used for converting the ultrasonic waves into an electric signal, and, by adding a time shift to the received signal waveform between the respective elements, both transmission and reception are generally focused within the object.
As described above, while a spatial resolution in the depth direction of approximately 0.13 mm can be realized by using the pulse-echo technique, higher spatial resolution is being demanded. For example, if it is possible to observe the layer structure of the vascular wall of the carotid artery in further detail, this may contribute to the early detection of arteriosclerosis or the like.
Non Patent Literature 1 shows the results of imaging the layer structure of the vascular wall by performing frequency domain interferometry (the FDI method), and the Capon method, which is adaptive signal processing. As a result of performing the Capon method by using a received signal and applying the FDI method, it is possible to further improve the spatial resolution in the depth direction (scanning line direction). However, it is assumed that there are a plurality of reflecting layers within the signal range (processing range) in the depth direction that was cut out for performing FDI processing. Moreover, it is likely that the plurality of reflected waves from adjacent reflecting layers will mutually have high correlation. When adaptive signal processing such as the Capon method is directly applied to the received signal of a plurality of reflected waves having high correlation as described above, it is known that unexpected operations such as the negation of intended signals tend to occur. In order to reduce the influence from signals having the foregoing correlation (correlative interference waves), the FDI method and the Capon method can be applied to the received signal of the reflected waves by additionally using the frequency averaging technique.
In addition, upon adopting the frequency averaging technique for a received signal of acoustic waves having a broad frequency band such as with pulse waves, whitening of the received signal is performed using a reference signal. Patent Literature 1 describes an ultrasonic probe capable of suppressing the sidelobe level by causing the backing material to have distribution.    Patent Literature 1: Japanese Patent Application Laid-Open No. H6-125894    Patent Literature 2: Japanese Examined Patent Publication No. H1-24479    Patent Literature 3: Japanese Examined Patent Publication No. H1-24480    Non Patent Literature 1: Hirofumi Taki, Kousuke Taki, Takuya Sakamoto, Makoto Yamakawa, Tsuyoshi Shiina and Toru Sato: Conf Proc IEEE Eng Med Biol Soc. 2010; 1: 5298-5301.
As described above, a reference signal is used in the adaptive signal processing to which the FDI method is applied. The closer this reference signal is to the actually acquired reflected waveform, the greater the effect of achieving higher spatial resolution based on the adaptive signal processing to which the FDI method is applied.
Nevertheless, in effect, with the acoustic wave pulse that is transmitted into the object, the waveform will change depending on the position of its arrival (reflected position). In particular, at positions of different depths, the waveform of the transmitted acoustic wave pulse tends to differ. Thus, there were cases where the effect of achieving higher spatial resolution based on the adaptive signal processing to which the FDI method is applied could not be sufficiently yielded.